1. Field of the Invention
The present invention relates to ink jet printing apparatus and more specifically to improved constructions and procedures in such apparatus that effect an air flow, for protecting the print head assembly from contamination, that is induced by an operative printing function(s) of the apparatus.
2. Description of the Prior Art
The term "continuous" has been used in the field of ink jet printer apparatus to characterize the types of ink jet printers that utilize continuous streams of ink droplets, e.g. in distinction to the "drop on demand" types. Continuous ink jet printers can be of the binary type (having "catch" and "print" trajectories for droplets of the continuous streams) and of the multi-deflection type (having a plurality of print trajectories for droplets of the continuous streams). Binary type apparatus most often employs a plurality of droplet streams while multi-deflection apparatus most often employs a single droplet stream.
In general, the print head assembly of continuous ink jet printing apparatus includes an ink cavity to which ink is supplied under pressure so as to issue in a stream from an orifice plate in liquid communication the cavity. Periodic perturbations are imposed on the liquid stream (e.g. vibrations by an electro-mechanical transducer) to cause the stream to break up into uniformly sized and shaped droplets. A charge plate is located proximate the droplet breakoff point and imparts electrical charge in accord with a print information signal to effect selective droplet deflection in accord with the charge borne by the droplet. A catcher is provided to catch non-printing droplets.
Certain elements of such a print head assembly, e.g. the orifice and charge plates, should be of relatively minute scale so as to provide good printing resolution. These elements must be fabricated and mounted very precisely in order to achieve quality printing. Thus dust particles are repugnate to the nature of the print head assembly; and a serious problem is presented by large volumes of print media moving in very close operative relation with the print head assembly. In other ink jet printing apparatus, e.g. drop on demand printers, it is desirable to protect at least the orifice structure from dust particles.
The most successful prior art approaches to this problem involve forcing clean (e.g. filtered) air into the region of the critical print head elements to prevent unwanted dust and debris from reaching those elements. However, the prior art approaches for providing the protective air flow have certain disadvantages. First, the external blowers used to provide such a protective air flow add cost, size, energy usage and noise to the printing apparatus. When the print head assembly is a moving part, it is difficult to maintain a uniform air flow from the blowers to the print head assembly; and it is highly desirable, from the viewpoint of accurate droplet placement, that the air flow conditions around the droplet path be quite stable. That is, ink droplets are very small and influenced in trajectory by low-velocity air currents so that uniform air flow, in a direction generally parallel to the droplet flight path is highly desired for optimum print quality.